29 Jesus answered, “The work of God is this: to believe in the one he has sent.”
Typical of humans, the crowd asked Jesus what THEY must do to get to Heaven. The human mindset never changes does it? Heaven should be given to us as the result of something WE do - (like fast, sacrifice, accept, believe, or repent).
But you have to read Jesus’ reply in the plain language. In reply to their question Jesus did not say, “To do the works God requires, accept the one whom He sent.” Oh, that would have been so easy and would have cleared up everything between Arminians and Calvinists, but in fact He said the exact opposite. Jesus immediately turned it around. He turned it away from what man must do and He identified what God DOES. Not what God can do or what God may do, but what God DOES. He does it sovereignly without asking the permission of those He has chosen.
The work of God is this..., NOT the work of man, but the work of God. The Work of God is this: that you believe in the one He has sent.
So it’s very simple: if a man does not believe, God has NOT done a work in him. If a man does believe, it’s because God HAS done a work in him. For we are GOD’S workmanship, not ours, ... (Ephesians 2:10), God began a work in us, we didn’t start it (Philippians 1:6). The fact that we believe in Christ is proof God has worked in us. Without God working in us, we would NOT believe, we would NEVER believe. Lost man loves the darkness and hates the Light and WILL NOT come into (believe) the Light. Those were Jesus’ own words (John 3:20).
Bottom Line: Believing on Jesus is NOT something a lost man is capable of doing. According to John 6:29, God FIRST works in him, making him believe.